The present invention is generally directed to a bridge and more particularly, to an improved pre-stressed, concrete bridge having both internal and external tensioning tendons.
Engineers are consistently striving to build bridges that are stronger, lighter, and capable of spanning longer distances while having improved durability and lower costs. Increasing the distance that a bridge may span without supporting piers or increasing the distance between supporting piers is also very desirable. To accomplish the above goals, engineers over the years have moved from stone and wood bridges to iron and steel bridges, to reinforced concrete bridges and more recently to pre-stressed concrete bridges.
Reinforced concrete bridges are generally poured in place around a rebar or other reinforcing members. As concrete, under its own dead weight may generate huge compressive forces, some reinforced concrete bridges experience downward creep which could lead to eventual failure of the bridge. To prevent downward creep, the length of distance span by reinforced concrete bridges is limited. To solve these problems, engineers developed pre-stressed concrete bridges and over the years technology for pre-stressed concrete bridges has developed to three main methods for pre-stressing the concrete.
The first method is to cast the concrete around an already pre-stressed tendon or tendons. This method works well for pre-cast concrete members that are then shipped to the site of the bridge and lifted individually into place. Although more difficult to implement due to the difficulty in locating the anchors to provide tension, this method may also be used by bridges that are cast in place on site. The method of casting concrete around already prestressed or pretensioned tendons provides an excellent bond between the tendon and the concrete. This bond increases the resistance to corrosion of metal tendons and allows as the concrete adheres and bonds to the tendons direct transfer of tension from the tendon to the concrete. If the tension on the tendon is released, the tension is transferred to the concrete by static friction minimizing any problems with release of tension on the tendon. This method also allows precasting of pieces without anchors that maintain tension during shipment and lifting into position. Precasting pretensioned pieces allows higher levels of quality control due to the ability to control the curing process as discussed below. Therefore, the bridge elements are usually pre-cast or pre-fabricated off site and then transported to the bridge site. Transportation difficulties may limit the size of the pre-cast portions thereby limiting the distance that the concrete bridge may span without supporting piers. Another problem with the above method is that the tendons usually limited to a pathway in the concretes of a straight line, due to the tension being pre-applied before the concrete is cast around the tendons. Applying tension to the tendons before casting of the concrete limits the pathways that the tendon may follow and thereby limits the availability of additional strength through various other pathways. Attempts to modify the pathway from a straight line typically cause other problems, which usually detract from the structural capabilities of the cast concrete member.
The second method is to cast a pathway into the concrete for the tendon to be later passed through. This method is typically known as bond post-tension concrete. More specifically, the concrete is cast around a hollow member that creates a tendon passageway. The hollow member may be formed from a variety of materials such as plastic, steel or aluminum. While the hollow members are typically formed in a straight line, in some instances they may be curved or have other shapes or pathways. While the concrete is typically cast without the tendon inserted into the hollow member, in some instances the casting occurs with the tendon inserted into the passageway member which may provide for easy manufacturing as the tendon may be difficult to insert in certain non-straight pathway configurations. After the concrete is cast, tension may be applied to the tendons. The concrete may be cast in place at the site of the bridge or pre-cast and lifted in sections into place. This method is commonly used in pre-tensioned concrete bridges.
The third method is called unbonded post-tension concrete. In unbonded post-tension concrete, the concrete is cast around the tendons which are not tensioned which also allows for the tendons to be able to form non-straight pathways. In place of a passageway member, the tendons are generally coated with a low friction material such as lithium grease and sheaved generally plastic sheeting formed by an extrusion process. As with bonded post-tensioned concrete bridges, anchors and other devises to secure tension are required. Some bridges use a combination of the above methods.
Some bridges in place of internal tensioned tendons in the concrete have used external tensioned tendons. Traditionally external tendons were not desirable as they were subject to corrosion and required regular maintenance, such as painting of steel cables to maintain integrity of the cables. External tendons typically ran under transverse beams and then upward on each end to anchor the tendons. The problem with using external tendons beyond the lack of durability for steel tendons was that the available pathways are limited and the external tendons are very weak in the transverse or lateral direction compared to the internal tendon.
While the above methods provide improvement in bridges and allow for greater spans to be made without supporting piers, it is desirable to further improve upon post-tension concrete and the above methods to create a cost-effective bridge that is easy to assembly while improving the durability, lowering the maintenance costs, and allowing the greater spans of distances in a cost-effective manner.